resolve_ref(): explicitly fail if a symlink is not readable

Previously the failure came later, after a few steps in which the
length was treated like the actual length of a string.  Even though
the old code gave the same answers, it was somewhat misleading.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -518,6 +518,8 @@ const char *resolve_ref(const char *ref, unsigned char *sha1, int reading, int *
 		/* Follow "normalized" - ie "refs/.." symlinks by hand */
 		if (S_ISLNK(st.st_mode)) {
 			len = readlink(path, buffer, sizeof(buffer)-1);
+			if (len < 0)
+				return NULL;
 			if (len >= 5 && !memcmp("refs/", buffer, 5)) {
 				buffer[len] = 0;
 				strcpy(ref_buffer, buffer);